This invention relates to semiconductor devices, and more specifically relates to a novel method for manufacture of a plurality of semiconductor devices from a common wafer.
The initial steps in the manufacture of power semiconductor devices such as controlled rectifiers or the like normally take place in a wafer fabrication facility in which a very large diameter wafer has junctions formed therein for a plurality of identical devices. After the formation of the junctions in the large wafer, the individual devices are separated from the wafer and are then further processed separately, usually in an assembly facility. In the further processing, an expansion plate contact is first alloyed to the bottom of the individual wafer elements. Thereafter contact metals are applied to the upper surface of the individual wafer elements. This contact metallizing process normally requires masking and oxide-etching for each individual wafer element.
The above sequence of first alloying the expansion contact and later applying contact metals has been necessary because the contact metals used in power devices are commonly aluminum. The aluminum contact metal would diffuse into the wafer surface at the alloying temperatures used for applying the expansion contact to the bottom of the wafer and would interfere with the diffused junction pattern.
After metallizing, the outer periphery of the individual wafer element is tapered to increase the breakdown voltage of the device. This process employs either an acid etch process or a grinding process followed by an acid etch to remove the damage caused by the grinding. However, the aluminum contact can be attacked by the acid etch used in the beveling operation. Therefore, it was necessary to protect the metallizing by coating it, for example, by gold plating and by wax, prior to the acid etch operation.
All of the above steps were carried out in an assembly area on individual wafer elements which must be separately handled. Thus, the added steps and separate handling substantially increase the expense of the device and reduce process yield.